The use of standby power supplies containing a battery and an inverter for maintaining A.C. electrical power to computers during failure of the power lines voltage is well known. It is highly desirable that the changeover operation be carried out as quickly as possible in order to avoid the loss of the contents of volatile memory. Ideally, such changeover systems should be very quick-acting, typically in the approximate range of one millisecond, since the filter capacitors of the onboard power supply of the computer are not designed to supply power for more than a time of the order of one-quarter of the line voltage waveform period, i.e., approximately 3 or 5 milliseconds.
In addition to the requirement for rapid reaction time for achieving the changeover from line power to battery power, there is the necessity for a power line voltage fault detector which will give a rapid and unambiguous reaction to power line abnormalities mandating system changeover from normal power line operation ("line mode") to battery-powered operation ("battery mode"). In addition to total power line voltage failure, such abnormalities also include transient and relatively long-term line overvoltage and undervoltage conditions as well.
There are two general approaches known in the art for providing such emergency backup power. One employs a battery-powered inverter permanently connected to the load and having a battery charger connected into the power lines, and in normal operation providing a charging rate at least equal to the discharge rate of the battery at maximum inverter output power. In such systems the fault detector is used to disable the battery charger, typically by totally disconnecting it from the input power lines in case of dangerously high line voltages. The inverter will continue to run and supply power for as long as adequate charge remains in the battery. Such a system, however, requires that the battery charging circuit must be able to deliver to the battery at least as much power as the maximum rated output power of the inverter. This typically mandates an additional high wattage transformer, greatly increasing the cost and weight of the system.
An alternative approach, which is described detail in the instant application, uses the inverter output transformer to charge the battery from appropriate taps thereon when the system is in line mode, and which disconnects the power lines from the output terminals during battery mode, at which time the inverter is switched on by the fault detector to an active condition to provide power to the load for a period of at least several minutes. In the event that the load is a digital computer, this time interval gives the operator time to store volatile memory contents in non-volatile storage, and further gives him time to make sure that the system is properly shut down with no disc recording heads in transit. Power failure under such circumstances can frequently result in damage to magnetic memory discs. U.S. Pat. No. 3,389,268, issued to Jamieson et al, U.S. Pat. No. 4,366,389, issued to Hussey and U.S. Pat. No. 4,400,625, issued to Hussey show representative prior art inverter systems wherein the inverter transformer is permanently connected across the output terminals.
One basic problem with these latter systems is that provision must be made to provide electrical power to the control switching circuitry and the fault detector in those cases where the battery is completely dead. Provision must be made to provide such power from the input power lines without exposing such circuitry to a burn-out situation in case the power line voltage goes to a catastrophically high value. At least one case is known wherein the neutral line of a Y-connected power distribution system was inadvertently broken, resulting in massive damage to a number of digital computers being fed therefrom.
It is desirable that the inverter transformer be permanently connected to the output terminals so that in normal line operation, the flux phasing will be proper when a transition to battery operation is made. Such systems are known in the art, and normally employ an oscillator to drivingly energize the inverter, the oscillator in turn being phase locked with the incoming electrical line voltage and being disabled from this phase-locked condition to free-run at a frequency very close to the normal line frequency in battery mode. Attendant to the line-to-battery transition the oscillator synchronization is disabled, and suitable switching circuitry is enabled so that the inverter is synchronously excited with respect to the flux phasing in the transformer.
Such a system has a concomitant necessity for a rapid disconnecting of the power supply output terminals from the power line input terminals, and requires that the inverter excitation be switched on almost immediately after this cut-out operation occurs. Additionally, when the fault detector senses restoration of normal line voltage, the inverter must be brought into phase lock again with the power line, after which time the inverter must be disabled immediately prior to operation of the input-output switching system to line mode so that normal line operation may again be resumed. This switchover operation is normally timed to be done in the vicinity of an axis crossing of the power line voltage, so that during the brief period wherein no power is delivered to the output terminals the filter capacitors of the power supplies associated with the loads can maintain adequate internal voltage during this transition period.
To accomplish these properly timed switchovers recourse may be made to silicon controlled rectifiers of the type which can be immediately turned off when in a conducting condition. If the power supply is designed to provide many hundreds of watts, or possibly a kilowatt, the current demands of the triggering circuitry which extinguishes such silicon controlled rectifiers is extremely high, and this requires that the system power supply and certain associated circuitry be designed with these very large currents in mind. This seriously increases the cost of such a power supply.
Another problem that the fault detector must contend with arises when the power distribution line system has substantial internal resistance. In particular, if the power supply is actuated to battery mode for significant period of time, resulting in a significant run down of the battery voltage, and hence the output voltage, then upon restoration of normal line operation, a high transient inrush current will be experienced, this transient current representing the charging transient of various power supply capacitors not only in the load, but in the power supply as well. This can cause an oscillatory condition to occur by pulling the power line input voltage below the low-voltage triggering threshold of the fault detector, resulting in an undesirable series of retriggerings between line and battery mode until stable system operation is achieved. Finally, inductive transients associated with system changeover from one mode to the other must not cause serious triggering of the fault detector.
There remains a need for a standby power supply system which addresses all of the above problems in a cost-efficient way.